This application claims priority from Korean Patent Application No. 2002-39154, filed on Jul. 6, 2002, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates to a proton-conducting polymer, and more particularly, to a proton-conducting polymer with good mechanical properties and protonic conductivity and capable of effectively suppressing cross-over of methanol, a preparation method thereof, a polymer membrane manufactured using the proton-conducting polymer, and a fuel cell using the polymer membrane.
2. Description of the Related Art
Recently, with growing concerns about the environment and the exhaustion of energy resources, and the commercialization of fuel cell automobiles, there is an urgent need for the development of reliable, high-performance fuel cells that are operatable at an ambient temperature with high-energy efficiency and for the development of polymer membranes capable of increasing the efficiency of fuel cells.
Fuel cells are new power generating systems that convert energy produced through the electrochemical reactions of fuel and oxidative gas directly into electric energy. Such fuel cells can be categorized into fuel cells with molten carbonate salt, which are operable at a high temperature of 500-700° C., fuel cells with phosphoric acid, which are operable around 200° C., and alkaline electrolyte fuel cells and solid polymer electrolyte fuel cells operable between room temperature and 100° C.
SPE fuel cells include proton-exchange membrane fuel cells (PEMFCs) using hydrogen gas as a fuel source and direct methanol fuel cells (DMFCs) which generate power using liquid methanol directly applied to the anode as a fuel source.
SPE fuel cells, which are emerging as the next generation of a clean energy source alternative to fossil fueled engine, have high power density and high-energy conversion efficiency. In addition, SPE fuel cells are workable at an ambient temperature and are easy to hermetically seal and miniaturize, so they can be extensively applied to the fields of zero emission vehicles, power generating systems for house use, mobile telecommunications equipment, medical equipment, military equipment, equipment in space, and the like.
The basic structure of a PEMFC as a power generator producing a direct current through the electrochemical reaction of hydrogen and oxygen is shown in FIG. 1. Referring to FIG. 1, a PEMFC has a proton-exchange membrane 11 interposed between an anode and a cathode. The proton-exchange membrane 11 is formed of an SPE with a thickness of 50-200 μm. The anode and cathode includes respective anode and cathode backing layers 14 and 15 for supplying reaction gases, and respective catalyst layers 12 and 13, where oxidation/reduction of reaction gases occur, forming gas diffusion electrodes (hereinafter, the anode and cathode will be referred to as “gas diffusion electrodes”). In FIG. 1, reference numeral 16 denotes a carbon sheet having gas injection holes and acting as a current collector.
As hydrogen as a reactant gas is supplied to a PEMFC having such a structure described above, hydrogen molecules decompose into protons and electrons through oxidation reaction in the anode. These protons reach the cathode through the proton-exchange membrane 11. Meanwhile, in the cathode, oxygen molecules take the electrons from the anode and are reduced to oxygen ions. These oxygen ions react with the protons from the anode to produce water.
As shown in FIG. 1, in the gas diffusion electrodes of the PEMFC, the catalyst layers 12 and 13 are formed on the anode and cathode backing layers 14 and 15, respectively. The anode and cathode backing layers 14 and 15 are formed of carbon cloth or carbon paper. The surfaces of the anode and cathode backing layers 14 and 15 are treated for reaction gases and water to easily permeate into the proton-exchange membrane 11 before and after reaction.
DMFCs have a similar structure to the PEMFC described above, but use liquid methanol instead of hydrogen as a fuel source. As methanol is supplied to the anode, an oxidation reaction occurs in the presence of a catalyst to generate protons, electrons, and carbon dioxide. Although DMFCs has lower energy efficiency than PEMFCs, the use of liquid fuel in DMFCs makes their application to portable electronic devices easier.
Ion-conducting polymer membranes are mostly used for a proton exchange membrane interposed between the anode and the cathode of fuel cells. Polymers for ion-conducting polymer membranes requires high ionic conductivity, electrochemical stability, acceptable mechanical properties, thermal stability at working temperatures, the possibility of being processed into low-resistant thin films, and smaller degree of swelling when soaking up liquid, etc. Fluorinated polymer membranes having fluorinated alkylene in their backbone and sulfonic acid groups at the terminals of fluorinated vinylether side chains, such as Nafion by Dupont, are currently available for ionic-conducting membranes. However, such fluorinated polymer membranes are unsuitable for automobile fuel cells due to the high price and cause problems of cross-over of methanol and lower performance when used for DMFCs.
To address for the problems, there has been intensive research on a variety of polymers capable of giving electrolyte membranes suitable electrochemical properties and thermal stability. Representative examples of such polymers include heat-resistant aromatic polymers, such as polybenzimidazole, polyethersulfone, polyetherketone, etc. However, these aromatic polymers are too rigid to dissolve and to be processed into thin films.
U.S. Pat. No. 6,245,881 discloses a fuel cell with a sulfonated polyimide ion-conducting membrane. However, this sulfonated polyimide ion-conducting membrane has poor mechanical strength because it is manufactured through direct sulfonation to the polyimide backbone, which originally has a degree of mechanical strength. The poor mechanical strength of the sulfonated polyimide ion-conducting membrane causes difficulties in subsequent processes involved in the manufacture of fuel cells, for example, in the manufacture of a membrane-electrolyte assembly (MEA).